This invention relates to an electronic scanning radar apparatus, and especially to a FM-CW type of electronic scanning radar apparatus for use on vehicle.
FIG. 1 is a time-chart showing transmitted signals and received signals and a principle of mixing in a FM-CW radar and FIG. 2 is a block diagram showing a conventional FM-CW type of electronic scanning radar apparatus.
On-vehicle radars for measuring distance, speed and azimuth with respect to a preceding target which exists in a forward direction have been developed in order to control an automotive vehicle crash prevention and tracking traveling.
A FM-CW radar system is used far measuring distance and relative speed with respect to a preceding target since this system has a simple structure of a signal processing circuit. As shown in FIG. 1, a signal S1 which frequency is linearly changed is transmitted from a transmission antenna in the FM-CW system. A signal S1 which is reflected by a target is received as a signal S2, and the received signal S2 and the transmitted signal S1 are mixed, with each other as shown in FIG. 1, thereby generating a beat signal S3 which component is a frequency difference between the received signal and the transmitted signal (beat frequency fb). This beat frequency is proportional to a reciprocating propagation delay time Δt from a target, and distance can be computed therefrom.
When measuring azimuth, an electronic scanning system for scanning all azimuths in a short time is employed. In the electronic scanning system, a reflected wave from a target is received by a plurality of antenna elements (array antenna) which are arranged according to some rule. And, a time difference occurs which is determined by the azimuth of the target to each antenna, a position where each antenna is arranged and the frequency of the received signal between channels of the received data. The azimuth of the target can be detected by this time difference (or difference of phases). As such kind of method, Digital Beamforming (DBF) is known. In DBF, the azimuth can be detected in such a manner that the received data is converted into digital data with an AD converter, and thereafter a correlation between each channel and vector data (array manifold vector) is taken (see document “Adaptive signal processing by array antenna” which has been published on 1998 by Kagaku Gijyutsu Shuppan written by Nobuo KIKUMA).
As mentioned before, data simultaneously received by a plurality of antenna elements are necessary in the electronic scanning system. But, in such a structure that the AD converter is prepared for each antenna element, the whole machine is made complex and expensive. Then, the structure as shown in FIG. 2 having a switcher 7 arranged between each antenna element 6 and an AD converter 13 in order to receive data, dividing time has been proposed.
Besides, phase information between the antennas is necessary in order to obtain the azimuth. In a conventional method, processing with FFT (Fast Fourier Transform) is executed by a FFT-type phase detector 15 as shown in FIG. 2 (see Japanese patent application publication number of which is 2000-284044).
Problems at the time of extracting phase information with FFT are as follows. Firstly, a weak reflected wave signal from a target is hidden from a strong reflected wave from, a target. Secondly, an accuracy of the phase information is uniquely determined by broadening of a main lobe of FFT, that is, by sampling frequency and number of sampling, and it is difficult to detect at such an accuracy depending on a distance to a target that distance accuracy has a priority in a short distance, and azimuth accuracy has a priority in a long distance. In order to obtain the phase information so as to give a priority to the distance accuracy in a short distance and give a priority to the azimuth accuracy in a long distance with FFT, it is necessary to change the sampling frequency and the number of sampling in a hardware, and a circuit structure is made complex, thereby.
Then, an object of the invention is to provide a n electronic scanning radar apparatus for extracting phase information at a high accuracy and for adjusting distance-azimuth resolution in order to solve the above-mentioned problems.